Such a sheet metal machining center is used to cut e.g. pieces of arbitrary shapes off a metal sheet. The first carriage, which may be called the carriage `Y`, moves along the guides in the frame with regard to the frame. This path may be called the direction of the Y-axis. The second carriage, which may be called the carriage `X`, moves along the guides in the first carriage in a direction perpendicular to the path of the first carriage. The path of the second carriage may be called the direction of the X-axis. The coordinates of the movement of the first carriage are calculated from a certain zero point on the frame, and the coordinates of the movement of the second carriage from a zero point in the memory of a CNC control center. Both carriages are moved mechanically with a driving mechanism working on the principle of screw/nut, pinion/rack or other such thing.
The sheet to be machined is fastened to the second carriage with clamps in the second carriage. The clamps may be operated e.g. mechanically or magnetically or in any other suitable way with which the sheet can be held immobile with regard to the second carriage. Thus, the sheet can be moved along the paths of both the second and the first carriages, i.e. within a sort of X/Y coordinates. The X-coordinates of the sheet are calculated from the top pin in the frame, the stop pin being movable to the upper and lower positions in the direction perpendicular to the plane of the work table. The Y-coordinates of the sheet are calculated from the front edge in the end of the clamp element.
A numeric data processor (CNC-control) controls the movements of the carriages to different positions and the operation of the whole machine. The CNC-control center is informed about the carriage position by e.g. a pulse sensor attached to the driving mechanism of the carriage or by a position sensor attached to the carriage. These devices and their operations are part of well-known techniques in the field and, hence, are not described here in more detail.
For machining of different sheets the positions of the clamps with regard to the second carriage must often be changed, because the dimensions of the sheet blooms vary or the sheet must also be machined at preceding locations of one or more clamps.
In the second carriage there is a guide along which the clamps can be moved in the direction of the path of the second carriage with regard to the second carriage. This way the positions of the clamps on the edge of the sheet can be rechosen considering the new circumstances. The clamps can be manually locked immobile in the second carriage and released from this locking.
The sheet is usually cut with a punching tool, e.g. a bayonet/pad tool. The sheet is moved with regard to the tool on the X/Y coordinates e.g. so that the bayonet follows the edge of the pattern to be cut. The bayonet can also punch separate holes in the sheet on spots chosen on the X/Y coordinates. The machine has a rotating table, or a so-called tool revolver, where different tools are kept and from where the machine automatically exchanges the needed tool for the machining position. The sheet can also be machined by a laser or plasma cut or with a chip cutting tool, such as a drill.
In the art, the repositioning of the clamp from the first position to the second position is done manually by detaching the clamp locking in regard to the carriage and moving the clamp to the second position and then locking the clamp there. To determine the position of the clamp with regard to the second carriage, there is a measuring scale in the second carriage and a mark line in the clamp. The readings of the measuring scale increase away from the stop pin. When the clamp or the clamps are fastened to the sheet while the sheet leans on the stop pin, the second carriage must be in such a position that its range of path in direction X suffices for the job, or in a so-called loading position. There can be several loading positions. The only prerequisite is that the path allowance of the second carriage suffices for the job. The basic loading position is the loading position where the zero point of the measuring scale of the second carriage coincides with the zero point of the X-coordinate of the sheet. The clamp positions in readings of the measuring scale are provided by the designer. The described method takes a relatively long time. This is a considerable drawback, because all through the set-up time the machine is idle. Since, however, the trend is toward shortening set-up times so that even very small production series might be economically processed in a sheet metal machining center, the length of the set-up time of the clamps has been a considerable drawback.